This invention more particularly applies to the optical control of structures made up of distinct uniform zones whose optical properties vary according to the length of the lighting wave. More particularly, the invention applies to the optical control of integrated circuits between certain production stages relating to these circuits. Inside an integrated circuit, each uniform zone in fact consists of, for example, silicon layers deposited on the substrate.
To obtain the image of a sample, it is known that it is generally necessary to use a light source which lights the sample and an optical system forming the image of this sample. For controlling microelectronic integrated circuits, the optical system used is a microscope. Recording of the image is effected at the output of the optical system (a microscope concerning the application in question) by means of an image sensor, such as a photographic plate for example, or a
camera which supplies on its outputs recording signals corresponding to the image. Moreover, this camera may be a charge transfer type camera (known as a CCD camera--Charges Coupled device).
The contrast obtained in the image between the various uniform zones of the sample depends on the selected lighting type and in particular on the spectrum of wavelengths of this lighting in relation to the sample optical properties (reflection or transmittance).
It is extremely important, especially as regards the production control of integrated circuits, to obtain the best available contrast between two zones, for example, of an image of this circuit so as to be able to detect any possible production defects appearing more particularly at the limit of these zones.
Generally, control is effected by comparing a recorded image of the circuit to be controlled and the corresponding recorded image of a circuit regarded as perfect.
The simplest and most currently used means in micro-electronics in order to obtain images displaying the various zones of a circuit is effected by means of reflected bright field microscopy. Lighting of the object is effected through the lens of a microscope under quasi-normal incidence on the circuit to be controlled.
The storing of images is effected in real time, for example by means of a video camera, namely a CCD type, coupled to acquisition electronics transferring the image into a data processing memory.
The defects are detected from the numerical difference between the numerical signals corresponding to the image acquired from the circuit to be controlled and from numerical signals corresponding to a corresponding reference image of a circuit regarded as faultless.
In these conditions, knowing that most of the production defects are constitued by inclusions from one zone into a nearby zone, it is obvious that obtaining in this case a good contrast between the various zones facilitates the detection of faults.
To optimize this contrast, a wavelength filtering is carried out of the light source which lights the sample. Contrast optimization is understood to mean determining the desired contrast, the latter not necessarily being maximum.
Currently, there is no automatic method making it possible to select a wavelength filtering of the source light, optimized according to the sample to be examined.